PMD insensitive direct-detection optical OFDM systems using self-polarization diversity

ABSTRACT

A self-polarization diversity technique to combat PMD in a direct-detection optical OFDM system. This technique does not require any dynamic polarization control, and can simultaneous compensate PMD in a WDM system with one device. Simulation results show that this technique virtually completely eliminates the PMD impairments in direct-detection optical OFDM systems.

This is a continuation of application Ser. No. 11/856,002, filed Sep.14, 2007 now U.S. Pat. No. 7,860,406.

FIELD OF THE INVENTION

This invention relates generally to the field of optical communicationsand in particular to apparatus and methods that employ self-polarizationdiversity to compensate polarization mode dispersion (PMD) indirect-detection optical orthogonal frequency division multiplexing(OFDM) systems.

BACKGROUND OF THE INVENTION

Orthogonal frequency division multiplexing (OFDM) has been widelyemployed in RF wireless communication systems such as wireless cellularsystems, digital audio and video broadcasting systems due to itsdesirable spectral efficiency, easy implementation, and robustness tomulti-path propagation and phase distortion. Recently OFDM has beenproposed for use in optical communication systems for example, to combatmodal dispersion in multimode fiber and chromatic dispersion in singlemode fiber.

As is known, there are two types of optical OFDM. One isdirect-detection optical ODFM, which uses optical intensity modulationand direct-detection, and the other is coherent optical OFDM, whichrequires optical IQ modulation and optical coherent detection.

As the symbol rate of an OFDM signal is very low, polarization modedispersion (PMD) does not cause any significant inter-symbolinterference (ISI) in an optical OFDM system. Unfortunately however, PMDcauses the state of polarization (SOP) to change with frequency. ThisSOP misalignment between sub-carriers and carrier induces signal fading,resulting in performance penalties for an optical OFDM system.

To compensate this effect, polarization diversity has been proposed forcoherent optical OFDM systems (See, e.g., W. Shieh, W. Chen and R. S.Tucker, Electron Lett., vol. 42, no. 17, 2006). Fortunately, with adirect-detection optical OFDM system, the PMD penalties can be reducedby aligning the SOP of the carrier with the sub-carriers—as in asub-carrier multiplexing system. Unfortunately however, this methodrequires dynamic polarization control and cannot completely eliminatethe PMD impairments as the SOP misalignment among sub-carriers is notcorrected.

SUMMARY OF THE INVENTION

An advance is made in the art according to the principles of the presentinvention whereby a self-polarization diversity technique is used tocompensate PMD in a direct detection optical OFDM system. In sharpcontrast to the prior art, the present invention does not employ dynamicpolarization control while substantially eliminating PMD impairments ina direct-detection optical OFDM system.

According to an aspect of the invention an optical signal is received ata receiver where it is split into two independent optical signals. Oneof the independent signals is directed to a direct-detection opticalOFDM receiver. The other one of the independent signals is firstseparated into optical carrier and sub-carrier components, the SOP ofthe optical carrier is rotated by substantially 90 degrees and thecarrier and sub-carriers are then recombined and subsequently directedinto another direct-detection optical OFDM receiver. The processedsignals are then recombined and directed into a demodulator.Advantageously, PMD effects are substantially eliminated and there is noneed for dynamic polarization control as prior art PMD compensatorsutilized. Finally, this technique may be used in a wavelength divisionmultiplexed (WDM) system for all channels while employing only a singlereceiver device.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the present invention may be realizedby reference to the accompanying drawings in which:

FIG. 1 is a schematic of a direct-detection optical OFDM withself-polarization diversity according to the present invention;

FIG. 2 is diagram of the SOP of: FIG. 2(A) carrier and sub-carriers atpositions A and B at receiver shown in FIG. 1, and FIG. 2(B) carrier andsub-carriers at position C at receiver shown in FIG. 1;

FIG. 3 is a series of constellation diagrams showing: FIG. 3(A) beforephase correction; FIG. 3(B) after phase correction; and FIG. 3(C) withpolarization diversity; where DGD=50 ps, and stars, circles and dots arefor polarization angles of 0, π/4, and π/3 respectively;

FIG. 4 is a series of graphs showing: FIG. 4(A) BER vs. OSNR with andwithout 1^(st) order PMD in worst case; and FIG. 4(B) BER vs. inputpolarization with order PMD of 50 ps at 11-dB OSNR PoID with and withoutpolarization diversity;

FIG. 5 is a graph showing samples of BER in a link with 100 ps averageDGD at OSNR=11 dB.

DETAILED DESCRIPTION

The following merely illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the invention and are includedwithin its spirit and scope.

Furthermore, all examples and conditional language recited herein areprincipally intended expressly to be only for pedagogical purposes toaid the reader in understanding the principles of the invention and theconcepts contributed by the inventor(s) to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the invention.

PMD Effects on Direct-Detection Optical OFDM System

In OFDM, signals are transmitted through a large number of orthogonalsub-carriers. The OFDM baseband signal is written as:

$\begin{matrix}{{{s(t)} = {\sum\limits_{k = {- \infty}}^{\infty}{{w\left( {t - {kT}_{s}} \right)}{\sum\limits_{i = 1}^{N_{sc}}{C_{i,k} \cdot {\exp\left\lbrack {{j2}\;\pi\;{f_{i}\left( {t - {iT}_{s}} \right)}} \right\rbrack}}}}}},{T_{s} = {T_{fft} + T_{g} + T_{w}}},} & (1)\end{matrix}$where C_(i,k) is the information of the kth OFDM symbol at the ithsub-carrier, N_(sc) is the number of sub-carriers, T_(s), T_(fft),T_(g), and T_(w) are the OFDM symbol period, effective part time, guardtime and windowing time, respectively, f_(i)=(i−1)/T_(s) is thefrequency of the i-th sub-carrier, and w(t) is the windowing function.

Those skilled in the art will readily appreciate that the guard time isused to preserve the orthogonality among sub-carriers when there ispartial symbol overlap induced by dispersion, and the windowing functionto reduce the out-of-band spectrum. For a direct-detection optical OFDMsystem—at the transmitter—the complex baseband OFDM signal is convertedto a real signal by modulating the real and imaginary components onto aRF-carrier. This signal is then converted to an optical signal byoptical intensity modulation.

To understand how PMD affects direct-detection optical OFDM systemperformance, we assume that the signal is linear polarized and there isonly 1^(st)-order PMD. At the receiver side, the Jones vector of thecarrier and sub-carriers are represented as [cos θexp(−jω₀Δτ/2), cosθexp(jω₀Δτ/2)]^(T) and [cos θexp(−jω_(i)Δτ/2), cosθexp(jω_(i)Δτ/2)]^(T), respectively, where ω₀ and ω_(i) are angularfrequencies of the carrier and sub-carriers, Δτ is the differentialgroup delay (DGD), θ is input polarization angle, and superscript T isvector transpose.

The mixing term after the photodetector is expressed as:f(Δω_(i))=cos² θexp[−j(ω₀−ω_(i))Δτ]+sin² θexp[j(ω₀−ω_(i))Δτ].  (2)Those skilled in the art will readily recognize that equation (2) showsthat PMD has two effects on optical OFDM system. More particularly, whenthe signal is aligned with a principal state of polarization (PSP) (θ=0or π/2), PMD induces phase shift, and when it splits equally between thetwo PSPs. f(Δω_(i))=cos(Δω_(i)Δτ/2), resulting in signal fading.

In a general case, there are both phase shifting and signal fading. Thephase shift can easily be corrected by the “1-tap equalizer” in the OFDMreceiver, but the signal fading cannot be equalized in amplifierspontaneous emission (ASE) noise limited systems, as increasing signallevel also increases noise and thus no improvement can be obtained.

Direct-Detection Optical OFDM System with Self-Polarization Diversity

The schematic of a direct-detection optical OFDM system withself-polarization diversity is shown in FIG. 1. The transmitter—shown inthe top portion of FIG. 1, is a conventional direct-detection opticalOFDM transmitter. A receiver—constructed according to the presentinvention—is shown in the lower portion of FIG. 1.

When operational—at the receiver—a received signal is equally split intotwo portions through the effect of a splitter. As shown in FIG. 1, thesplitter is a 3 dB 50/50 splitter. Those skilled in the art willappreciate that splitting ratios other than 50/50 are possible alongwith characteristics other than 3 dB.

With simultaneous reference to FIG. 2(A) and FIG. 2(B) there is shown inFIG. 2 (A) the SOP of carrier and sub-carriers at positions A and 13 ofthe receiver shown in FIG. 1, while FIG. 2(B) shows the carrier andsub-carriers at position C of the receiver shown in FIG. 1. Thoseskilled in the art will of course recognize the polarizationmisalignment cause by PMD with respect to the optical carrier and OFDMsub-carriers shown in FIG. 2(A).

Continuing with the discussion of the receiver, one portion of the splitsignal goes to a direct-detection optical OFDM receiver. The other partgoes to a circulator. A fiber Bragg grating (FBG) terminated with aFaraday rotator mirror is connected to port 2 of the circulator.Advantageously for our purposes, the FBG only lets the carrier passthrough and reflects sub-carriers.

The Faraday rotator mirror rotates the carrier in such a way that itsoutput SOP is orthogonal to its input SOP, no matter what the input SOPis. Therefore, the SOPs of sub-carriers at positions 13 and C of FIG. 1are the same, while the carrier's SOP at these two positions areorthogonal to each other.

As those skilled in the art will readily appreciate, in this manner,polarization diversity is achieved without any dynamic polarizationcontrol. If the free spectral range (FSR) of the FBG is the same aschannel spacing in a WDM system, the single device can be used in theWDM system to achieve the self-polarization diversity simultaneously forall the channels. The two parts are combined after the single-tapequalizer of the OFDM receiver and sent to the sub-carrier demodulationto restore data.

Simulation Results

To evaluate the present invention, a 10-Gb/s direct-detection OFDMsystem constructed according to the present invention may be understoodthrough the use of simulations. A symbol period of 25.6 ns, guard timeof 800 ps and window time of 800 ps are used in the simulations. Thereare 240 sub-carriers, which are modulated with quadrature phase shiftkeying (QPSK). The baseband OFDM signal occupies 5-GHz bandwidth, whichis modulated onto an RF-carrier of 6 GHz using an I-Q modulator.

This signal is then modulated onto an optical carrier with a linearoptical modulator. At the output of the modulator, a single-side band(SSB) filter removes one side-band and attenuates the carrier to makethe same power in the optical carrier and sideband. A 3^(rd)-orderGassssian optical filter with a 20-GHz 3-dB bandwidth is used at thereceiver to reject the ASE noise.

FIG. 3 shows the effects of O-order PMD on the received constellations.PMD causes both phase shift and signal fading. The phase shift can beeasily corrected by the equalizer, as shown in FIG. 3 (B). Thepolarization multiplexing completely eliminate the signal fading, asillustrated in FIG. 3 (C).

Turning now to FIG. 4, there is shown the effects of 1^(st)-order PMD onbit error rate (BER) of the direct-detection optical OFDM according tothe present invention both with and without self-polarization diversity.To calculate BER, 1000 OFDM symbols are used. The optical signal tonoise ratio (OSNR) is defined as the ratio of signal power (includingcarrier and sub-carriers) to ASE noise power in 0.1-nm bandwidth. Itclearly illustrates that PMD causes big penalties in thedirect-detection optical OFDM, but with the self-polarization diversity,PMD effects are virtually eliminated.

FIG. 5 shows the performance of the self-polarization diversity in thepresence of all-order PMD. Average DGD of 100 ps is used in the figure,and results of 500 PMD samples are given. It shows that BER of thedirect-detection optical OFDM has a large fluctuation due to PMD (from7.3e-3 to 0.35) when there is no self-polarization diversity, whereaswhen the self-polarization diversity is used, the PMD induced BERfluctuation is negligible.

At this point, while the present invention has been discussed anddescribed using some specific examples, those skilled in the art willrecognize that the teachings are not so limited. Accordingly, theinvention should be only limited by the scope of the claims attachedhereto.

1. A method of receiving an optical signal, the method comprising:splitting an orthogonal frequency division multiplexed (OFDM) opticalsignal into a first portion and a second portion; directing the firstportion to a first direct-detection OFDM receiver; directing the secondportion to a circulator; rotating one of a carrier or sub-carriersportion of the second portion such that states of polarization of theone of the carrier or sub-carriers portion as measured before and afterthe circulator are orthogonal while states of polarization for the otherof the carrier or sub-carriers portion of the second portion remain thesame before and after the circulator; directing output of the circulatorto a second direct-detection OFDM receiver; combining outputs of thefirst and second direct-detection OFDM receivers; and directing thecombined outputs to a demodulator for demodulation of data.
 2. Themethod of claim 1 wherein said splitting exhibits <5 dB loss.
 3. Themethod of claim 2 wherein said splitting is performed by an asymmetricsplitter.
 4. The method of claim 2 wherein said splitting is performedby a symmetric 50/50 splitter.
 5. The method of claim 1 wherein saidrotating of the one of the carrier or sub-carriers portion is providedby an effect of an optical filter terminated with a Faraday rotatormirror.
 6. The method of claim 5 wherein said optical filter is a fiberBragg grating (FBG) filter.
 7. The method of claim 1 further comprising:demodulating the combined outputs.
 8. The method of claim 1 wherein saidreceiving of the optical signal occurs at a receiver.
 9. An apparatusfor receiving an optical signal, the apparatus comprising: a splitterfor splitting an orthogonal frequency division multiplexed (OFDM)optical signal into a first portion and a second portion; a firstdirect-detection OFDM receiver for detecting the first portion; acirculator for rotating one of a carrier or sub-carriers portion of thesecond portion such that states of polarization of the one of thecarrier or sub-carriers portion as measured before and after thecirculator are orthogonal while states of polarization for the other ofthe carrier or the sub-carriers portion of the second portion remain thesame before and after the circulator; a second direct-detection OFDMreceiver for detecting output of the circulator; and a combiner forproducing a combined output based on output of the firstdirect-detection OFDM receiver and second direct-detection OFDMreceiver.
 10. The apparatus of claim 9 wherein said splitter exhibits <5dB loss.
 11. The apparatus of claim 10 wherein said splitter is anasymmetric splitter.
 12. The apparatus of claim 10 wherein said splitteris a symmetric 50/50 splitter.
 13. The apparatus of claim 9 wherein saidcirculator includes an optical filter and a Faraday rotator mirror. 14.The apparatus of claim 13 wherein said optical filter is a fiber Bragggrating (FBG) filter.
 15. The apparatus of claim 9 further comprising ademodulator for demodulating data based on the combined output.
 16. Anapparatus for receiving an optical signal, the apparatus comprising: asplitter for splitting an orthogonal frequency division multiplexed(OFDM) optical signal into a first portion and a second portion; a firstdirect-detection OFDM receiver for detecting the first portion; acirculator for rotating a carrier portion of the second portion suchthat states of polarization of the carrier portion as measured beforeand after the circulator are orthogonal while states of polarization fora sub-carriers portion of the second portion remain the same before andafter the circulator; a second direct-detection OFDM receiver fordetecting output of the circulator; and a combiner for producing acombined output based on output of the first direct-detection OFDMreceiver and second direct-detection OFDM receiver.
 17. The apparatus ofclaim 16 wherein said splitter is an asymmetric splitter.
 18. Theapparatus of claim 16 wherein said splitter is a symmetric 50/50splitter.
 19. The apparatus of claim 16 wherein said circulator includesan optical filter and a Faraday rotator mirror.
 20. The apparatus ofclaim 19 wherein said optical filter is a fiber Bragg grating (FBG)filter.
 21. The apparatus of claim 16 further comprising a demodulatorfor demodulating data based on the combined output.